update doc to use `tablecloth.column.api` rather than `tech.v3.datatype.functional`

daslu#14823mo ago

good first issuehelp wanted

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Issue Description

In the doc, there are a couple of places where tech.v3.datatype.functional is used. Probably now, tablecloth.column.api can be encouraged instead.

Issue #148 — scicloj/tablecloth | QAForge